1. Field of the Invention
The present invention relates to a wireless communication, and more particularly, to an apparatus and method for estimating a location of a mobile station in a wireless local area network.
2. Discussion of the Related Art
In a wired LAN environment, detection of collision can be obtained from a potential variation of a transmitting medium. Yet, in a wireless LAN environment, it is difficult to detect collision on a transmitting medium (e.g., air, etc.). Hence, a multiple access scheme for a wireless transmission by avoiding collision in advance is called CSMA/CA (carrier sense multiple access with collision avoidance) that is a medium access control scheme used by CSMA/CA wireless LAN. The collision avoidance means the algorithm including the steps of if a circuit is vacant before transmitting data of its own, creating random numbers and then waiting by the corresponding value. A transmission is initiated only if a transmission medium is determined as not in use after a randomized standby time.
By MAC algorithm generally used by wireless LAN, a signal for acknowledgment is transmitted to prepare for a collision despite an absence of a data transmission. If it is confirmed that the acknowledgement signal has been transmitted without collision, data is then sent. In IEEE 802.11 standard, a procedure called ACK or CTS/RTS is used. In doing so, an operating principle of CSMA/CA is explained as follows.
First of all, a transmitting end sends an RTS (request to send) signal to a receiving end. In response to the RTS signal, the receiving end sends a CTS (clear to send) signal to the transmitting end. The transmitting end sends data to the receiving end. Finally, the receiving end sends an ACK signal to the transmitting end in response to the data reception.
The collision avoidance scheme in IEEE 802.11 standard is schematically described as follows.
IFS (Inter Frame Space)
First of all, although it is confirmed that a channel is in an idle state, collision is avoided by delaying a transmission. The transmission is not performed right after the channel in the idle state has been found. Instead, it is necessary to wait by prescribed duration called IFS. This is because a remote station may have begun a transmission despite that a detected channel looks as if staying in the idle state. In this case, if the transmission is performed without waiting, a collision may occur. Hence, it is necessary to wait during the IFS. Moreover, in CSMA/CA, IFS may be used to regulate a priority of a station or frame. In IEEE 802.11 standard, such an IFS as DIFS (Distributed IFS), PIFS (PCF IFS), SIFS (Short IFS), EIFS (Extended IFS) and the like is used.
A mobile station, which operates using the CSMA/CA scheme, performs a following procedure to transmit data. First of all, a mobile station A senses a carrier to check whether a mobile station B is transceiving data (i.e., whether the mobile station B is transmitting electric waves). If the mobile station A is aware that the mobile station is transceiving the data, the mobile station A waits until DIFS becomes vacant. In the vacant state, the mobile station A waits for randomly determined duration. If the vacant state continues, the mobile station A starts a transmission.
Contention Window
A contention window is a predetermined time divided into time slots. A station ready for a transmission selects a random number and then waits for the selected random number. In this interval, the station detects a channel after each time slot. In doing so, if the station detects that a channel is already in use, the station just stops a timer instead of restarting the corresponding process. Thereafter, if it is detected that the channel is in an idle state, the station activates the timer again. Therefore, a station having waited for the longest time has a priority.
However, in the current IEEE 802.11 standard mentioned in the above description, a detailed method of measuring a location of a mobile station in Wireless LAN has not been proposed. Thus, the demand for a detailed method for a WLAN using mobile station to estimate position information rises and the present invention intends to propose the corresponding method.